Vibrating screen cloths (1) are used in so-called “shakers” for catching and vibrating out laterally that portion of the returning drilling mud or drilling liquid which is constituted by cuttings, sand and metal particles, and letting the drilling mud as such pass filtered through the screen cloth. The drilling mud is led over a series of one or more screen cloths (1), the coarser first, which removes the coarser particles, and later over finer and still finer screen cloths (1). Examples of such screen cloths arranged in frames are shown in FIG. 1.
The entire screen assembly is vibrated in order to make drill cuttings and particles of all sizes to migrate out across the edge of the shaker screen frame so as for the drilling liquids liquid fraction and finer particles, below the shaker screen's mesh size will pass through the screen cloth. There are also screen cloths (1) arranged as an endless band which are arranged for rotating slowly. A shaker screen may comprise a rectangular main frame (2) of about 1×1 m2 and an example is shown in that the main frame may have 10 rows each of 20 rectangular oblong cell frames (22) each supporting its portion of a screen cloth. Most shaker screen frames (2) comprise an upper screen cloth (1) with the desired mesh size, e.g. from 2500 μm (2.5 mm) for the very coarse screen, and down to 20 μm (0.020 mm), all with a backing of a support cloth with mesh size of between 2500 micrometers (2.5 mm) or 2000 μm (2.0 mm) or even 1000 micron (1.0 mm) which are all rather rough and durable. The screen cloth (1) and the support cloth (11) may be fixed to the cell frames (22) by means of adhesives or hardening cement mass so as for the screen cloth and the support cloth not to mutually vibrate and grind and thus to prevent that the screen cloth from be worn over the cell frame, and further prevent that the support cloth from being worn against the cell frame.
The so-called “cut point” of the screen cloth is defined by the size of the particles which have a given probability for being removed in a filter. Usually cut points may be indicated such as “D50 500” and “D90 500” defining 50% and 90% probabilities of being removed for particles of a given size, here 500 gym. For particles larger than the indicated “cut point” the probability for the particle to be sorted out is larger. For particles os a size less than the indicated “cut point” the probability is larger to pass through with the mud.